1. Field of the Invention
The present invention relates to a duplexer which is a transmitting/receiving separate type filter for use in UHF band mobile communication. In particular, the present invention relates to a duplexer having dual coupled line characteristics.
2. Description of the Prior Art
The duplexer which is used as a transmitting/receiving separate type filter in the UHF band mobile communications uses TEM mode dielectric coaxial resonators. The number of the coaxial resonators is decided depending on the filter requirement characteristics but generally, in order to increase the signal damping for the transmitting frequency at the band pass filter of the receiving terminal, the number of the resonators of the filter of receiving terminal is made larger than that of resonators of the band pass filter of the transmitting terminal.
FIG. 1 illustrates the constitution of a duplexer using the conventional dielectric. In this duplexer, an integrated type is provided in which a plurality of resonators are connected to a single dielectric block. That is, the duplexer includes: four resonators 2 of the transmitting terminal, five resonators 2' of the receiving terminal, and a resonator 3 used as a separating circuit for separating the transmitting/receiving signals.
In a dielectric block 1, the resonators 2, 2' and 3 having holes of the same size are formed from an upper face 1' of the dielectric block to its bottom. Further, the five faces of the dielectric block 1 excluding the upper face 1' are electroplated. Therefore, the resonators form short circuits connected from the bottom of the dielectric block to the ground. The upper face 1' functions as a 1/4 resonator which is an open end terminal. Further, the diameters of the resonators are uniform, and therefore, they are uniform impedance resonators in which the impedance of the open terminal is same as that of the short circuit terminal.
Further, in the first resonator and in the last resonator, there are inserted conductive rods 5 as input/output terminals. Further, dielectric sleeves are fitted to the input/output terminal conductive rods 5 for a capacitive coupling between the conductive rods 5 and the resonators 2. Further, resonator 3 is provided for the branching circuit so as to match the impedance between the transmitting terminal and the receiving terminal at an antenna terminal 5'. Therefore, input signals which are inputted into the transmitting filter are not transferred to the receiving filter, but are transmitted through the antenna terminal. The signals which are received through the antenna terminal are transferred to the transmitting filter but are transferred to the receiving terminal.
In this duplexer described above, the coupling between the resonators is done through a single coupling line in which the odd and even mode admittances of the open terminal and short circuit terminal are constant. Further, all the faces of the dielectric block except the upper face 1' are electroplated with a metal.
In the duplexer of FIG. 1, if an electric equivalent circuit is illustrated by using a coupling line for the transmitting filter, then it becomes as shown in FIG. 6. In FIG. 6, the equivalent circuit includes three resonators, and a UIR resonator is shown in the form of a short-circuited 1/4-wavelength resonator 19. Further, the coupling between the resonators is shown in the form of a distribution device 20 based on an induced coupling. Further, the coupling between the input/output conductive rods and the resonators is shown in the form of capacitance 21.
In this duplexer, if the insertion loss characteristics are illustrated for the band pass filters used in the transmitting terminal and the receiving terminal, then it becomes FIG. 9 in which the dampings at the frequencies higher or lower than the pass band are almost same.
However, in the mobile communication, in order to efficiently utilize the frequency, the transmitting band and the receiving band are closely positioned. Further, in order to make the damping of the adjacent frequency large, the transmitting band pass filter requires a more superior damping characteristic at a frequency higher than that of the pass band. Further, the band pass filter which is used at the receiving terminal requires a higher damping characteristic at a frequency lower than that of the pass band. In this band pass filter, in order to improve the damping characteristics, if the number of the resonators is increased, the damping characteristics are improved, but the insertion loss is increased, and the bulk of the filter is increased. Accordingly, there is required a pole filter having a blocking pole without transmitting the signals at a particular frequency, and without increasing the number of resonators.
FIG. 2 illustrates another conventional duplexer in which short-circuited 1/4-wavelength individual resonators 6 and a concentrated device are used, thereby providing a duplexer having a polarity. A transmitting filter includes three individual resonators, and a chip capacitor 9 is inserted into between a first resonator and input terminal 7. The coupling between the resonators is carried out in such a manner that an electrical pattern is connected on a PCB board 8, and that an external chip capacitor 9 is connected to it. In order to block the transmission of signals at a particular frequency, a separate chip inductor 10 is inserted into an open terminal of a second resonator. The resonance circuit is modified by connecting the chip inductor 10 to the open terminal of the second resonator so as to form a pole frequency. That is, at a frequency at which the impedance of the second resonance circuit becomes zero, the signals from the input side cannot be transferred, but flows through the resonance circuit to the ground, and therefore, there occurs a pole frequency. Such a pole frequency is generated at a frequency higher than the pass band of the transmitting filter, and therefore, the damping of the receiving signals can be increased. The filter of the receiving terminal includes four individual resonators, while a chip capacitor is inserted into between the last resonator and an output terminal. An equivalent circuit for this duplexer is illustrated in FIG. 7.
The equivalent circuit of FIG. 7 includes three resonators. The coupling between the resonators is done in such a manner that an electrical pattern is connected to a PCB board 8, and an external chip capacitor 9 is connected to it. In order to block the transfer of signals at a particular frequency, a separate chip capacitor 10' is disposed at an open terminal of a second resonator. Under this condition, the pole frequency occurs at a frequency lower than that of the pass band of the receiving frequency, and therefore, the damping of the transmitting frequency can be increased.
The frequency characteristics of this filter is illustrated in FIG. 10. In order to match the impedance of the filters of the antenna terminal 7', the transmitting terminal and the receiving terminal, there are used a chip inductor 11 and a chip capacitor 11'. Therefore, the signals which are inputted into the transmitting input terminal are not transmitted to the filter of the receiving terminal due to the impedance adjusting inductor, but are propagated through the antenna terminal to the external. The signals which are received to the antenna terminal are transmitted to the filter of the receiving terminal, thereby transferring energy.
In such a duplexer, a pole filter is formed, and therefore, the damping characteristics can be improved with a small number of resonators. However, external devices such as a chip capacitor or a chip inductor are used, and therefore, the bulk of the product is increased, as well as complicating the manufacturing process.
In the transmitting/receiving separate type filter of the mobile communication, a high damping characteristic is required, and owing to the compactness and light weight trend of the terminals, the filter and duplexer have to be miniaturized. Therefore, a duplexer which is small in size and superior in the damping characteristics is in demand. However, in the conventional duplexer, either the number of the resonators is increased, or an external device such as a chip capacitor or a chip inductor has to be used. Consequently, the size of the filter is increased, and the manufacturing process becomes complicated. Therefore, a duplexer which is superior in the damping characteristics and small in size is in demand.
Recently in the mobile communications, in order to efficiently utilize the frequency, the transmitting and receiving bands are closely approached, while the miniaturization and light weight of the terminals are progressing at a fast pace.